An organic electroluminescence device (hereinafter, referred to as organic EL device) can be classified by the emission principle into two types: a fluorescent EL device and a phosphorescent EL device. When a voltage is applied to the organic EL device, holes are injected from an anode and electrons are injected from a cathode. The holes and the electrons are recombined in an emitting layer to form excitons. According to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In a fluorescent EL device which uses emission caused by singlet excitons, the limit value of an internal quantum efficiency is believed to be 25%. This is about 5% in terms of external quantum efficiency. Even considering technical improvement in efficiency for obtaining emission, in a blue-emission fluorescent device exhibiting CIEy=about 0.1, the limit value of the external quantum efficiency was 8%. A voltage applied at the maximum efficiency was about 4 to 6 V.
In association with the technology for bringing efficiency to the fluorescent EL device, several technologies are disclosed in which emission is obtained from triplet excitons, which have heretofore been not utilized effectively. For instance, a paper published in the Journal of Applied Physics, 102,114504 (2007) analyzed a non-doped device, in which an anthracene compound is used as a host. A mechanism is found that singlet excitons are generated by collision and fusion of two triplet excitons, whereby fluorescent emission is increased. Such a phenomenon in which singlet excitons are generated by collision and fusion of two triplet excitons is hereinafter referred to as TTF (Triplet-Triplet Fusion) phenomenon.
SID10 DIGEST, 560 (2010) discloses a blue-emission fluorescent OLED in which a layer of an aromatic compound (efficiency-enhancement layer, referred to as EEL) is interposed between an emitting layer including a host and a dopant and an electron transporting layer. It is reported that an OLED in which a compound EEL-1 is used as EEL is driven by a low voltage, exhibits a high external quantum efficiency and has a long lifetime compared with an OLED in which BPhen or BCP is used as EEL. This EEL can serve as a blocking layer for causing a TTF phenomenon. In BPhen and BCP disclosed in SID10 DIGEST, 560 (2010), a TTF ratio was very low and a voltage at 10 mA/cm2 was 4.5 V or more.